Thank you for contacting support.
Please note that earlier HTML 5 Editor was provided just to demonstrate the implementation of API features in your application. However it has been discontinued and we are not maintaining it for any fixes. However, the issue may be occurring because of already set margins of your source PDF file. You may change margins of respective page before adding the signature, with the code snippet below:
document.Pages[PageNumber].PageInfo.Margin = new MarginInfo(0, 0, 0, 0);
Please also note that the basic measuring unit in Aspose.PDF for .NET is point, where 1 inch = 72 points and 1 cm = 1/2.54 inch = 0.3937 inch = 28.3 points.
In case you still face the problem then please share a narrowed down code snippet along with source and generated PDF files so that we may try to reproduce and investigate it in our environment.